- hipervisor wants to run lots of guests, but have no memory for everyone, so it takes pages from guests and puts on disk
- linux wants to run lots of programs, but have no memory for everyone, so takes pages from processes and puts on disk
- both activities are independent and can run at the same time
- hipervisor moves one page from a guest to swap disk
- guest wants to move the same page to his own swap disk, and it references the page to mark it as swapped out
- page is referenced, so hipervisor needs to get it back from disk. to do so, another page from another guest is moved out to ma…